The present invention relates generally to a f.multidot..theta. correction system, and more particularly to the electrical f.multidot..theta. correction system in a laser printer which removes by the electrical f.multidot..theta. correcting method picture distortion due to the uniform speed scanning by the electrical f.multidot..theta. correcting method.
In general, a laser printer is a kind of electrophotography system which can perform a high quality of high speed print (more than 20 sheets/minute in A4 size) by coupling the high precision optical system with the semiconductor laser as shown in FIG. 1. The construction of the optical system determines the laser output required in relation to the oscillation wavelength and the light-splitting sensitivity of the photosensitive element, and is influenced by the size of the apparatus and the print speed thereof.
Since, in the laser printer, the reception sensitivity of the laser by the photosensitive element is lowered at long wavelengths, lasers emitting visible light which is easily sensed by said photosensitive element have been nowadays widely applied, also a rotating polygon mirror which is a deflector for deflecting the beam in dependence upon the print information transmitted as the electric signal has been used as an attachment, and a correction system for raising the print quality has been used.
Furthermore, in an optical scanning system of the laser printer, it is known as the major technique that a linear scanning is performed to form the focus on the photosensitive element that scans the laser light at uniform speed. However, since in the above-mentioned technique the scanned beam through the rotating polygon mirror does not form a circular arc on the focal plane, a linear focal plane may not be formed on the photosensitive drum and the beam through the polygon mirror rotated at uniform speed is not scanned onto the scanned plane(of said drum), thereby resulting in the occurence of the severe distortion or deformation of the picture in printing.
Accordingly, in order to correct the above-mentioned defects, technical analysis the above-mentioned uniform speed scanning was required. Referring to FIG. 2 to put it concretely, there is shown schematically a scanning system in a printer. Assuming that in a general lens the focal distance or length is f and that the angle formed between the light ray and the light axis is .theta., the image height(H) of the object positioned on the infinite origin may be given by the following equation: EQU H=f.multidot.tan.theta. (1)
Thus, since the laser printer performs the scanning of the beam through the mirror rotating at constant angular velocity so that the height H of the image may be proportional to tan .theta., the greater the distance from the light axis, the higher the scanning speed, thereby resulting in distortion of the image. Therefore, as shown in FIG. 2, there is a necessity for correcting the rotating mirror to arrange the height of image on position of a constant times of .theta.. This is called the f.multidot..theta. correction. Such a conventional f.multidot..theta. correction has been optically performed by coupling a plurality of convex and concave lenses. Thus the f.multidot..theta. correcting lenses have been separately used which obtain the f.multidot..theta. characteristics by selecting the refractive index which can be obtained as if laser light reflected from the rotating polygon mirror is emitted from the curvature center of a sphere.
However, since between one and nine lenses for the f.multidot..theta. characteristic are used and this also requires high precision processing and a complex design technique, the conventional f.multidot..theta. correction system has the problem of not having the high precision due to the complication of controls and increased cost of production due to the expensive f.multidot..theta. correcting lens and the complication of construction.